Cytotoxic T-lymphocyte antigen-4 (CTLA-4) was found in 1987 to be a new member of the immunoglobulin superfamily, characterized by domains sharing key structural features with either the variable (V) or the constant (C) immunoglobulin domains (Brunet et al., Nature 328, 267-270). It was elucidated that CTLA-4 played a critical role in regulation of immune system (Keilholz, U., J Immunother 31, 431-439). CTLA-4 was reported to reduce T-cell activation by competing with CD28 for binding site of CD80/CD86 (Rudd et al., Immunol Rev 229, 12-26). Although CTLA-4 protects individuals from autoimmune diseases, it could also suppress anticancer immunity. To avoid the unwanted immune responses caused by CTLA-4 in cancer treatment, several approaches manipulating T-cell costimulatory pathway are being explored to enhance anticancer immune response. Therapy targeting CTLA-4 is one of the most advanced strategies and has revealed promising results in late stage clinical trials (Hodi et al., N Engl J Med 363, 711-723; Hodi, F. S., Asia Pac J Clin Oncol 6 Suppl 1, S 16-23; Weber, J., Oncologist 13 Suppl 4, 16-25; and Ribas, A., Oncologist 13 Suppl 4, 10-15). One of the monoclonal antibodies against CTLA-4, ipilimumab, had been granted approval by the FDA in March of 2011 for treatment of metastatic melanoma. In addition to metastatic melanoma, CTLA-4 antibodies are currently undergoing numerous clinical trials for the treatment of malignancies including, pancreatic cancer, colorectal cancer, hepatocellular carcinoma, lymphoma, hormone refractory prostate cancer, ovarian cancer and acute myeloid leukemia.
Program death-1 (PD-1) is a member of the CD28 superfamily which triggers negative signaling pathway upon binding to its ligands, program death ligand 1 and 2 (PD-L1 and PD-L2) (Riley, J. L., Immunol Rev 229, 114-125). The interaction between PD-1 and its ligands leads to inhibition of proliferation, cytokine production, and cytolytic function of T-cell, thereby exhausting T-cell and suppressing its immune response. The PD-1/PD-L pathway plays an important role in tolerance and immunity. It protects tissues and organs from immune-mediated damage. However, this pathway has been shown to be utilized by pathogens of chronic infection and tumors to suppress antimicrobial and anticancer immunity. Given immune-modulating activity of PD-1/PD-L axis, therapeutics targeting this pathway has been developed for treatment of diseases ranging from infections, autoimmunity to cancers (Weber, J., Semin Oncol 37, 430-439).
Although progresses had been made to modify immune response against infectious and malignant diseases by targeting CTLA-4, PD-1/PD-L1/PD-L2 and other immune-modulating proteins, new approaches for anticancer and anti-infectious treatments by enhancing immunity while avoiding suppression of immune responses are still needed.